U.S. patent number 4,143,273 [Application Number 05/786,358] was granted by the patent office on 1979-03-06 for variable collimator.
This patent grant is currently assigned to Ohio-Nuclear, Inc.. Invention is credited to John Covic, Thomas R. McBride, Joseph B. Richey.
United States Patent |
4,143,273 |
Richey , et al. |
March 6, 1979 |
Variable collimator
Abstract
An automatic variable collimator which controls the width and
thickness of X-ray beams in X-ray diagnostic medical equipment, and
which is particularly adapted for use with computerized axial
tomographic scanners. A two-part collimator is provided which
shapes an X-ray beam both prior to its entering an object subject
to radiographic analysis and after the attenuated beam has passed
through the object. Interposed between a source of radiation and
the object subject to radiographic analysis is a first or source
collimator. The source collimator causes the X-ray beam emitted by
the source of radiation to be split into a plurality of generally
rectangular shaped beams. Disposed within the source collimator is
a movable aperture plate which may be used to selectively vary the
thickness of the plurality of generally rectangular shaped beams
transmitted through the source collimator. A second or receiver
collimator is interposed between the object subject to radiographic
analysis and a series of radiation detectors. The receiver
collimator is disposed to receive the attenuated X-ray beams
passing through the object subject to radiographic analysis.
Located within the receiver collimator are a plurality of movable
aperture plates adapted to be displaced relative to a plurality of
fixed aperture plates for the purpose of varying the width and
thickness of the attenuated X-ray beams transmitted through the
object subject to radiographic analysis. The movable aperture
plates of the source and receiver collimators are automatically
controlled by circuitry which is provided to allow remote operation
of the movable aperture plates.
Inventors: |
Richey; Joseph B. (Shaker
Heights, OH), McBride; Thomas R. (Chardon, OH), Covic;
John (Wickliffe, OH) |
Assignee: |
Ohio-Nuclear, Inc. (Solon,
OH)
|
Family
ID: |
25138366 |
Appl.
No.: |
05/786,358 |
Filed: |
April 11, 1977 |
Current U.S.
Class: |
378/7; 378/150;
378/160; 976/DIG.430 |
Current CPC
Class: |
A61B
6/032 (20130101); G21K 1/04 (20130101); A61B
6/06 (20130101) |
Current International
Class: |
A61B
6/03 (20060101); A61B 6/06 (20060101); G21K
1/02 (20060101); G21K 1/04 (20060101); A61B
006/02 () |
Field of
Search: |
;250/513,512,511,445T,401,514 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Smith; Alfred E.
Assistant Examiner: Grigsby; T. N.
Attorney, Agent or Firm: Fay & Sharpe
Claims
The invention claimed is:
1. In a computerized axial tomographic scanner having a source of
radiation, a subject receiving area, radiation detection means, a
first collimator means having radiation from said source of
radiation passing therethrough, and a second collimator means
having radiation from said source of radiation passing
therethrough, the improvement comprising:
a first variable aperture means associated with said first
collimator means for varying the radiation passing
therethrough;
a second variable aperture means associated with said second
collimator means for varying the radiation passing therethrough;
and
control means for causing said second variable aperture means of
said second collimator means to coact with said first variable
aperture means of said first collimator means.
2. The apparatus as set forth in claim 1 wherein said control means
causes said first variable aperture means to vary at least one
dimension of at least one aperture and causes said second variable
aperture means to vary by a corresponding amount at least one
dimension of at least one aperture.
3. In computerized tomographic scanner, a variable collimator
means, comprising:
(a) a source of radiation emitting a beam;
(b) a subject receiving area having said beam passing
therethrough;
(c) a first collimator means disposed between said source of
radiation and said subject receiving area wherein said first
collimator means has first variable aperture means;
(d) radiation detection means in substantial axial alignment with
said beam of radiation;
(e) a second collimator means disposed between said subject
receiving area and said radiation detection means, wherein said
second collimator has second variable aperture means for varying
the amount of radiation received by said radiation detection means
from said source of radiation; and
(f) wherein said first variable aperture means of said first
collimator means cooperatively interacts with said second variable
aperture means of said second collimator means such that varying
the aperture of one of said collimator means produces a
corresponding variation of the aperture of the other of said
collimator means.
4. The apparatus as set forth in claim 3 wherein said first
collimator means includes a first linear array of apertures for
dividing said beam of said source of radiation into a plurality of
beams.
5. The apparatus as set forth in claim 4 wherein said second
collimator means includes a second linear array of apertures, one
corresponding to each aperture of said first linear array.
6. The apparatus as set forth in claim 5 wherein said first and
second variable aperture means vary at least one dimension of each
aperture of said first and second linear arrays of apertures,
respectively.
7. The apparatus as set forth in claim 6 wherein said first and
second variable aperture means vary each aperture of said first and
second linear array correspondingly in dimensions generally
transverse to said linear arrays.
8. The apparatus as set forth in claim 6 wherein said first and
second variable aperture means discretely vary at least one
dimension of each aperture of said first and second linear arrays
between preselected aperture dimensions.
9. The apparatus as set forth in claim 6 further including control
means operatively connected to said first and second variable
aperture means for varying at least one dimension of each aperture
of said first and second linear arrays, and selector means
operatively connected with said control means for selecting said at
least one dimension whereby at least one dimension of the radiation
beams can be remotely varied.
10. Control means for a first variable collimator having a first
movable aperture means and a second coacting variable collimator
having a second movable aperture means comprising:
(a) first drive means for displacing said first movable aperture
means;
(b) position responsive means having an output indicative of the
position of said first movable aperture means;
(c) an aperture selection means having an output indicative of a
selected corresponding aperture for said first and second variable
collimators;
(d) comparator means receiving the output of said position
responsive means indicative of the position of said first movable
aperture means and the output of said aperture selection means
indicative of a selected aperture;
(e) drive control means responsive to the output of said comparator
means for causing said drive means to displace said first movable
aperture means and
(f) second drive means operatively connected with said aperture
selection means for displacing said second movable aperture
means.
11. In an apparatus for measuring the attenuation of radiation
after passage through a medium and for reconstructing a
representation of the medium, the apparatus including a source of
radiation mounted to pass radiation through the medium, a radiation
detection means on the side of the medium opposite the source,
means for causing a relative movement of at least the source of
radiation with respect to the medium in order that the radiation
passes through the medium along a plurality of paths along each of
which radiation is attenuated and the attenuated radiation detected
by the detection means, the improvement comprising:
a variable aperture means disposed between said source and said
detection means comprising:
a fixed shaping means having at least a first aperture therein for
shaping radiation passing therethrough into at least one beam, said
first aperture having plurality of dimensions for shaping the
cross-sectional dimensions of the beam;
a movable shaping means having at least a second aperture therein
mounted for displacement relative to said fixed shaping means, said
second aperture having at least one dimension smaller than said
first aperture, whereby the second aperture is movable to reduce at
least one cross-sectional dimension of the beam; and
control means operatively connected with said movable shaping means
for moving said movable shaping means at least between a first
position in which said second aperture is displaced from said first
aperture such that the radiation passes through said first aperture
to the exclusion of said second aperture, whereby the first
aperture shapes the cross section of the beam, and a second
position in which said second aperture is disposed adjacent said
first aperture such that the radiation passes through said first
and second aperture whereby the first and second apertures
cooperate to shape the cross-sectional dimensions of the beam.
12. The apparatus as set forth in claim 11 wherein said fixed
shaping means has a plurality of generally rectangular apertures
for shaping a plurality of beams having generally rectangular cross
sections defined by width and thickness dimensions.
13. The apparatus as set forth in claim 11 wherein said movable
shaping means is pivotally mounted whereby said control means
rotates said movable shaping means between the first and second
positions.
14. The apparatus as set forth in claim 11 further including a
second variable aperture means disposed between said source and
said detection means for variably shaping the cross-sectional
dimensions of the beam, said control means operatively connected
with said second variable aperture means.
15. In an apparatus for measuring the attenuation of radiation
after passage through a medium and for reconstructing a
representation of the medium, the apparatus including a source of
radiation mounted to pass radiation through the medium, a detector
means for the radiation on the side of the medium opposite the
source, means for causing a relative movement of at least the
source of radiation with respect to the medium in order that the
radiation passes through the medium along a plurality of paths
along which radiation is attenuated and detected by the detector
means, the improvement comprising:
a fixed shaping means having at least a first aperture therein, and
so positioned that radiation from the source passes through said
first aperture, said first aperture having a plurality of
dimensions for shaping the radiation passing therethrough into a
beam having corresponding cross-sectional dimensions;
a first movable shaping means having at least a second aperture
therein and so movably positioned adjacent said fixed shaping means
that said first and second apertures are at least partially
aligned; and
displacing means operatively connected with said first movable
shaping means for displacing said first movable shaping means
relative to said fixed shaping means to change the alignment of
said first and second apertures, whereby at least one
cross-sectional dimension of the beam is changed.
16. The apparatus as set forth in claim 15 wherein said fixed
shaping means further includes a plurality of grooves generally
saw-toothed in cross section disposed toward said source for
absorbing uncollimated radiation.
17. The apparatus as set forth in claim 15 wherein said movable
shaping means is displaceable between at least a first and a second
position, and wherein said second aperture includes at least one
projection so positioned that in said first position said
projection is out of alignment with said first aperture, and in
said second position said projection is in alignment with said
first aperture whereby the cross section of the beam is reduced in
at least one dimension as the movable shaping means is displaced
from said first position to said second position.
18. The apparatus as set forth in claim 17 wherein said fixed
shaping means has a plurality of apertures for shaping a plurality
of beams, and wherein said second aperture has a plurality of
projections, one corresponding to each of the plurality of
apertures and so positioned that in said first position one of said
projections is in alignment with each of said plurality of
apertures, and in said second position each of said projections is
out of alignment with any said plurality of apertures whereby the
cross section of the plurality of beams is reduced in at least one
dimension as the movable shaping means is displaced.
19. The apparatus as set forth in claim 18 wherein said plurality
of apertures in said fixed shaping means is arranged in at least
two substantially parallel linear arrays.
20. The apparatus as set forth in claim 15 further including a
second movable shaping means having at least a third aperture
therein and so movably positioned adjacent said fixed shaping means
and said first movable shaping means that said first, second and
third apertures are at least partially aligned, said displacement
means operatively connected with said second movable shaping means
for displacing said second movable shaping means relative to at
least said fixed shaping means.
21. The apparatus as set forth in claim 20 wherein said displacing
means displaces said first and second movable shaping means
relative to said fixed shaping means in opposite directions and by
equal spatial amounts.
22. The apparatus as set forth in claim 20 wherein said fixed
shaping means includes a first linear array of apertures, said
first movably shaping means includes a second substantially linear
array of apertures disposed substantially parallel with said first
linear array, said second movable shaping means includes a third
substantially linear array of apertures disposed generally parallel
to said first and second linear arrays, said first, second and
third linear arrays disposed in generally overlapping position, and
wherein said displacing means slides the second and third arrays
relative to the first array to vary the degree of overlap whereby
each beam is shaped by apertures in said first, second and third
arrays.
23. The apparatus as set forth in claim 20 wherein said displacing
means includes a screw thread means, a first screw thread follower
operatively connected to said first movable shaping means engaging
said screw thread means, a second screw thread follower operatively
connected to said second movable shaping means engaging said screw
thread means, and means for rotating said screw thread means
whereby the first and second screw thread followers are moved along
the screw thread means to move the first and second movable shaping
means.
24. The apparatus as set forth in claim 23 wherein said screw
thread means includes first and second screw threads engaging said
first and second screw thread followers respectively, said first
and second screw threads so pitched and rotated that said first and
second screw thread followers move equal distances in opposite
directions as the first and second screw threads rotate.
25. In an apparatus for measuring the attenuation of radiation
after passage through a medium and for reconstructing a
representation of the medium, the apparatus including a source of
radiation mounted to pass radiation through the medium, a detector
means for the radiation on the side of the medium opposite the
source, means for causing relative movement of at least the source
of radiation with respect to the medium to give an output fixed
shaping means having at least one substantially linear array of
apertures and so positioned that radiation from the source passes
through the apertures and is shaped into a corresponding array of
beams, each having a first dimension parallel to the linear array
and a centerline transverse to the linear array and positioned
centrally to said first dimension the improvement comprising:
a variable aperture means for simultaneously adjusting said first
dimension of each of said apertures symmetrically about said
centerline.
26. The apparatus as set forth in claim 25 wherein the variable
aperture means includes a first movable plate having at least one
linear array of apertures corresponding to the linear array of the
fixed shaping means, a second movable plate having at least one
linear array of apertures corresponding to the linear array of the
fixed shaping means and displacing means for displacing said first
and second plates generally equal distances relative to said
centerline in opposing directions.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to X-ray apparatus and more
particularly to a collimator structure therefor. Specifically, the
invention is concerned with an apparatus which automatically
controls the width and thickness of X-ray beams in X-ray diagnostic
equipment, and is particularly adapted for use with a device known
as a computerized axial tomographic scanner.
2. Description of the Prior Art
Collimators for controlling the size and shape of X-ray beams are
well known in the art and include those shown in Haupt U.S. Pat.
No. 2,542,196, Akaski et al U.S. Pat. No. 2,851,610 and Hura U.S.
Pat. No. 3,829,701.
Mechanisms known as collimators are commonly used to shape an X-ray
beam to a desired size and shape. In equipment used in radiographic
studies, these collimators generally include two pairs of
relatively movable diaphragms which shape an X-ray beam so as to
provide a rectangular cross-section. These diaphragms are also used
to eliminate two sources of extraneous radiation which may cause a
degradation of the image produced by the diagnostic equipment.
A first cause of extraneous radiation is commonly referred to as
scatter. In radiographic studies, it is desirable to confine the
X-ray beam to the area of the object under examination, not only to
minimize the exposure of the object and attending persons to the
primary beam, but also to minimize radiation scatter effects.
Radiation scatter is produced when the primary radiation beam
strikes an object and is defracted. If the size of the X-ray beam
is larger than required to accommodate a particular area under
investigation, the X-ray beam striking areas of the object around
the area of investigation will produce an unnecessary amount of
radiation scatter which has the effect of reducing the contrast of
the radiographic image.
A second source of extraneous radiation which may cause image
degradation is caused by what is known as the "penumbra" effect.
The X-ray beams are emitted from a very small area on an X-ray tube
anode known as the focal point. Theoretically, this spot can be so
small and bombardment of it with electrons so precise that the beam
is emitted in a precise and regular conical pattern of "on focus"
radiation. As a practical matter, however, the spot is a larger
area than a theoretically optimized spot and an X-ray tube emits a
penumbra or band of so-called "off focus" radiation from areas
around the spot. This penumbra or "off focus" radiation is another
source of image degradation.
Computerized axial tomographic X-ray scanners which provide the
reconstruction of a transaxial section of an object by means of
X-rays are also well known in the art as evidenced by Hounsfield
U.S. Pat. No. 3,778,614. A reconstructed image of an object can be
obtained by viewing an object via X-ray imaging from numerous
angles, mathematically reconstructing the detailed structure, and
displaying the reconstructed image. In general, X-ray beams are
passed through the object for detection by scintillation crystal
detectors. Analog outputs from these detectors go through signal
conditioning circuitry that amplifies, clips and shapes the
signals. A relatively simple analog to digital converter then
prepares the signals for the computer, which performs various
mathematical operations upon the data received and provides an
output which may be used to display the reconstructed image.
Prior art computerized axial tomographic scanners have generally
utilized fixed type collimators as evidenced by Hounsfield U.S.
Pat. No. 3,778,614, the disclosure of which is incorporated by
reference. In the case of a computerized axial tomographic scanner
having a plurality of radiation detectors, the conventionally used
collimator structure requires a rather complex apparatus which is
expensive to fabricate and greatly increases the bulk of that
portion of the scanner which is required to be moved, with the
attendant disadvantages.
Even further, the conventional collimator structure is difficult to
use because of its inflexibility. This inflexibility becomes
readily apparent when it is desired to use a different size or
shape X-ray beam during the scanning process. In order to realize
this feature with the conventional collimator structure, it is
necessary to entirely remove the collimator structure and replace
it with a collimator structure producing the desired size or shape
of X-ray beam. The expense and inconvenience of such a procedure
often makes it unfeasible to effect a change in the size or shape
of the X-ray beam used by the scanner.
There is, therefore, a need for a computerized axial tomographic
scanner having a variable collimator means which may be used to
readily change the size or shape of the X-ray beam utilized by the
scanner, which change may be accomplished with relative ease and in
a short period of time, and which provides a collimator means that
minimizes extraneous radiation in order to enhance image
quality.
The present invention solves this problem by providing an automatic
variable collimator structure which utilizes a two-part collimator
structure having movable aperture plates therein to effectively
change the size and shape of the X-ray beams utilized by the
scanner. The collimator structure additionally minimizes extraneous
radiation.
SUMMARY OF THE INVENTION
This invention relates to an automatic variable collimator which
controls the shape of X-ray beams in radiographic equipment. A
two-part collimator structure, which is comprised of a first or
source collimator and a second or receiver collimator, is utilized
to shape X-ray beams both prior to their entering the object
subject to radiographic analysis and after the attenuated beams
have passed through the object. Interposed between a source of
radiation and the object subject to radiographic analysis is the
first or source collimator which causes the X-ray beams emitted by
the source of radiation to be split into a plurality of generally
rectangular shaped beams. Within the source collimator is disposed
a movable aperture plate which may be used to selectively vary the
thickness of the plurality of generally rectangular shaped beams
transmitted through the source collimator. The movable aperture
plate is displaced by a motor means actuated by a control
circuitry. A second or receiver collimator is interposed between
the object subject to radiographic analysis and a series of
radiation detectors. The receiver collimator is disposed to receive
the attenuated X-ray beams passing through the object subject to
radiographic analysis. Within the receiver collimator are a
plurality of movable aperture plates which may be displaced by
motor means relative to a plurality of fixed aperture plates for
the purpose of varying the width and thickness of the attenuated
X-ray beams received from the object subject to radiographic
analysis. A first and second receiver collimator motor means may be
actuated by the control circuitry to provide a movement of the
movable aperture plates in order to cause the X-ray beams passing
therethrough to change their shape.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation of a transport frame used in a
computerized tomographic scanner having the collimator structure
mounted thereon, which collimator structure is shown in section to
more fully illustrate the details of construction thereof;
FIG. 2 is a top plan view of the source collimator having parts
thereof broken away and shown in section in order to more fully
illustrate details of construction thereof;
FIG. 3 is a front view of the source collimator having portions
thereof broken away and shown in section;
FIG. 4 is a partial sectional view of the source collimator shown
in FIG. 2 and taken along the lines 4--4 showing the details of
construction of the movable aperture plate;
FIG. 5 is a partial left side view with parts removed of the source
collimator shown in FIG. 3 disclosing the details of construction
of the return spring;
FIG. 6 is a front elevation of the second fixed aperture plate of
the source collimator;
FIG. 7 is a front elevation of the fixed aperture plate of the
source collimator;
FIG. 8 is a top view of the receiver collimator with portions
thereof broken away and shown in section to more fully illustrate
the details of the construction thereof;
FIG. 9 is a front sectional view of the receiver collimator shown
in FIG. 8 and taken along line 9--9;
FIG. 10 is a partial left sectional view of the transport frame and
collimator structure shown in FIG. 1 and taken along the line
10--10;
FIGS. 11 through 16 are front elevations of the various aperture
plates associated with the receiver collimator; and
FIG. 17 is a block diagram illustrating the control circuitry of
the exemplary embodiment.
DETAILED DESCRIPTION OF THE DRAWINGS
Shown in FIG. 1 is a computerized axial tomographic scanner
transport member 11 which is comprised of a movable frame member 12
which has fixedly attached thereto a source of radiation 32, a
source collimator 33, a receiver collimator 17, and a series of
receiver detectors denoted by the reference numeral 27. The source
of radiation 32, the source collimator 33, the receiver collimator
17, and the receiver detectors are all in substantial axial
alignment so that a beam of radiation emitted by the source of
radiation 32 will be directed through the source collimator 33 to
the receiver collimator 17 and finally received by the receiver
detectors 27. The path of radiation from the source of radiation 32
to the receiver detectors 27 is indicated by the lines designated
35 which represent X-ray beam paths. The beam width, as referred to
herein, is as indicated in FIG. 1 and is normally measured on the
center line of the scan circle 36.
It may now be appreciated that by selective transverse and
rotational movement of the transport member 11, the outputs of the
receiver detectors 27 are able to provide an output which may be
used by data processing means in the reconstruction of an image
representation of an object contained within the scan circle
36.
The source of radiation 32 emits X-rays in a general cone-shaped
configuration with the apex of the cone located at the focal spot
of an X-ray tube contained within the source of radiation 32, as
indicated by the X-ray beams 35. The cone angle is initially shaped
by the opening in the source of radiation housing and secondarily
by the safety shutter mechanism 31 contained within the source
collimator 33.
Further shaping of the X-ray beams is effected by aperture plates
29 and 37 in order to provide for the transmission through the
source collimator 33 a series of 12 rectangular beams. Further
shaping of the X-ray beams may be optionally performed by the
rotatable aperture plate 38, which may be rotationally disposed
within the beam path 35 in order to effect a shaping thereof so as
to reduce the thickness of the series of rectangular beams
transmitted through the source collimator 33.
Aperture means includes those structures which define an aperture.
For example, plate 29 and, in the position of FIG. 3, plate 38
surround and define each beam shaping aperture.
There is thus provided a means for subjecting an object within the
scan circle 36 to a series of 12 rectangular beams of radiation,
which beams may be optionally 1 of 2 thicknesses.
The X-ray beams transmitted through the source collimator 33 are
modulated and attenuated by objects within the scan circle 36 and
transmitted to the receiver collimator 17 which causes further
shaping of the X-ray beams and subsequently causes the X-ray beams
to impinge upon receiver detectors 27. The output of the receiver
detectors 27 may be processed to provide an image reconstruction of
the objects located within the scan circle 36.
The receiver detectors 27 are preferably high atomic weight
scintillators, such as those containing efficient radiation
detectors like sodium iodide, or calcium fluoride. Photomultipliers
may then be used as noiseless gain stages to convert the
scintillation light into a direct current which may be subsequently
used in the reconstruction of the image as is conventionally done
in computerized axial tomographic scanners.
Referring now to FIG. 2 of the drawings, there is disclosed the
source collimator 33 with parts broken away and shown in section to
more fully illustrate the details of construction thereof. The
source collimator 33 is generally comprised of a housing member 55
which, in the exemplary embodiment, is an aluminum casting. The
housing member 55 in cooperation with cover plate 56 defines an
X-ray collimator space within which are located the various beam
shaping members. Disposed within this space formed by the housing
member 55 and the cover plate 56 are interlocking lead plates,
designated 51 through 54, which are used to confine the travel of
X-ray beams through the source collimator 33 to a predetermined
path.
X-ray beams are emitted from the source of radiation 32 into the
collimator space through collimator space opening 57. A safety
shutter 31 having an aperture 46 therein is provided to control the
emission of X-ray beams into the remainder of the collimator space.
As shown in FIG. 2, the safety shutter 31 is in its closed position
wherein X-ray beams entering the collimator space opening 57 are
not allowed to be transmitted to the remainder of the collimator
space. When it is desired to introduce X-ray beams into the
remainder of the collimator space, the safety shutter 31 may be
rotated about its longitudinal axis by an electrically actuated
rotary solenoid 43 which is mechanically coupled to the shaft 44 of
the safety shutter 31 by means of a chain coupling 45.
It may now be readily seen that the rotary solenoid 43 may be
electrically actuated to impart a rotary displacement to the safety
shutter 31 via chain coupling 45 and safety shutter shaft 44 to
thereby cause the safety shutter aperture 46 to be disposed within
the collimator space so as to allow passage of X-ray beams
therethrough. The X-ray beam passing through the safety shutter
aperture 46 assumes the general size and shape of the aperture
opening in the shutter.
X-ray beams passing through the aperture in the safety shutter 31
impinge upon a first fixed aperture plate 37 which has an opening
therein of the general size and shape of the aperture 46 in the
safety shutter. The purpose of the first fixed aperture plate 37,
disclosed more fully in FIG. 7, is to effect the first stage of
X-ray beam shaping or collimation. In addition, the fixed aperture
plate 37 serves as a baffle to reduce the release of scatter X-rays
or secondary emission into the patient compartment.
At the exit end of the collimator housing 55 is a second fixed
aperture plate 29. The second fixed aperture plate 29, as more
fully illustrated in FIG. 6, is preferably made of a tungsten alloy
and has a series of 12 rectangular openings 47 through it. The
second fixed source plate 29 effects a second stage of beam shaping
by separating the X-ray beam into 12 smaller rectangularly shaped
beams. In order to reduce the emission of scattered and unusable
X-rays into the patient compartment, the second aperture plate 29
is placed as near to the periphery of the scan circle 36 and as far
from the first fixed aperture plate 37 as practical.
The beams of X-rays, as collimated at this point, are approximately
13 millimeters thick at the center line of the scanned circle.
In addition to the two fixed aperture plates 37 and 29, the source
collimator 33 contains a movable aperture plate 38 interposed
between the first fixed aperture plate 37 and the second fixed
aperture plate 29 and adjacent to the second fixed aperture plate
29. The movable aperture plate 38, as more clearly disclosed in
FIG. 4, is fixedly attached to a mounting bracket 48 which is in
turn fixedly attached to movable aperture plate shaft 49 and
rotates therewith. The rotating shaft 49 extends through the
collimator housing 55 and has affixed to one end thereof a return
spring 61. The return spring 61, as more fully shown in FIG. 5, has
one end thereof fixedly attached to the rotating shaft 49 and the
other end thereof fixedly attached to the housing member 55 in
order to spring bias the rotating shaft 49.
The other end of the rotating shaft 49 has fixedly attached thereto
a sprocket 62. The sprocket 62 is engaged with chain 63 which is in
further engagement with sprocket 64 which is fixedly attached to
the shaft 65 of the rotary solenoid 28. It may now be appreciated
that electrical actuation of the rotary solenoid 28 will cause the
shaft 65, the sprocket 64, chain 63, sprocket 62, and consequently
shaft 49 to rotate in response thereto and that upon removal of the
electrical actuation of the rotary solenoid 28, the rotating shaft
49 will tend to assume a "normal" position due to the spring
biasing afforded by return spring 61.
This arrangement permits the rotatable plate 38 to be rotated 90
degrees into a masking position adjacent to the second fixed plate
29 whereby the opening of the movable aperture plate 38, which is
smaller than the rectangular openings of the second fixed aperture
plate 29, will mask the upper and lower portions of the twelve
openings in the second fixed aperture plate 29, thereby effectively
reducing the aperture height to produce a series of beams which are
approximately 8 millimeters thick at the center line of the scan
circle. There is thus provided a means for reducing the thickness
of the X-ray beams transmitted through the source collimator 33.
Beam thickness, as used herein, is defined in FIG. 10 of the
drawings with the thickness normally being measured at the center
line of the scan circle.
In the exemplary embodiment, when the operator elects to reduce the
X-ray beam slice thickness from 13 millimeters to 8 millimeters, a
select switch is indexed which causes the energization of the
rotary solenoid 28 which is mounted on the collimator housing 55.
The rotary action of the solenoid 28 moves the movable aperture
plate 38 into a masking position adjacent to the fixed aperture
plate 29.
When the operator elects to change beam thickness from 8
millimeters to 13 millimeters, the select switch is changed to
de-energize the solenoid 28 and permit the spring 61 to return the
movable aperture plate 38 to its non-masking position. This
position is shown more clearly in FIG. 10 where the movable
aperture plate 38 is shown in solid lines in the masking position,
and is shown by phantom lines in its non-masking position. The
control circuitry for causing the selective energization of the
rotary solenoid 28 is described below in more detail.
Referring now to FIG. 8, there is shown a plan view of the receiver
collimator 17 with parts thereof broken away and shown in section.
The receiver collimator 17 is comprised generally of housing 71
which, in the exemplary embodiment, is an aluminum casting. The
housing 71 in cooperation with cover plate 72 defines a collimator
space in which are disposed the fixed and movable aperture plates
described below. Also situated in this collimator space are a
series of interlocking lead shield plates designated 73 through 76.
The interlocking shield plates 73 through 76 are designed to
confine the travel of X-ray beams through the receiver collimator
to a predetermined path.
To the front or source end of the housing 71 is affixed lead
scatter plate 16 which has a series of 12 rectangular openings
therein. These openings, as more fully illustrated in FIG. 11, are
slightly larger than the theoretical beam size at this point and
are not intended to influence beam size or shape. The face of the
scatter plate 16 has machined into it a series of concentric
grooves 81 forming generally a saw-toothed cross-section. These
grooves extend in all directions from the center of the plate 16 to
a distance far enough to encompass the maximum area of possible
X-ray penumbra released from the source collimator through the
patient compartment. The grooves 81 are intended to create
labyrinth effect for the purpose of absorbing uncollimated
X-rays.
Disposed within receiver collimator housing 71 adjacent to scatter
plate 16 is a fixed aperture plate 19 whose details of construction
are shown more fully in FIG. 12. The fixed aperture plate 19 has a
series of 24 rectangular openings through it arranged in two
separate rows of 12 openings each. The fixed aperture plate 19
serves two purposes. First, it divides what is at this point 12
long, narrow rectangular beams into 24 separate rectangular beams,
thereby creating two separate slices of information which are
contiguous at the center line of the scan circle. Second, the fixed
aperture 19 also determines beam thickness.
Adjacent to the fixed aperture plate 19 and disposed within the
collimator space is a movable aperture 21 whose configuration is
more fully disclosed in FIG. 13. The movable aperture plate 21 is
adapted to be laterally displaced relative to the fixed aperture
plate by means described below in more detail. The movable aperture
plate 21 contains a series of 12 rectangular openings in the same
relative location as those openings in the aperture plate 19. The
openings in the movable aperture plate 21, however, are slightly
larger than those openings in the fixed aperture plate 19 in order
to not affect the beam size. In addition, the movable aperture
plate 21 has a series of 12 smaller openings situated between the
larger rectangular openings and contiguous therewith. The placement
of these smaller openings between the larger openings creates a
saw-toothed type aperture as shown in FIG. 13.
When the movable aperture plate 21 is displaced along its length
relative to the fixed aperture plate 19 by means described in more
detail below, the protrusions 82 of the movable aperture plate 21
will cover the upper and lower portions respectively of the upper
and lower rows of openings in the fixed aperture plate 19, thereby
effecting smaller openings than existed when the large rectangular
openings of the movable aperture plate 21 were in alignment with
the openings of fixed aperture plate 19. The smaller openings
resulting from the fixed aperture plate 19 co-acting with the
movable aperture plate 21 produce a reduced beam width. It should
be noted that the movable aperture plate 21 does not alter the
location of the inner edge of the X-ray beams, i.e., the edge
nearest to the central line and, therefore, does not affect the
contiguousness of two slices of X-ray beams.
The movable aperture plate 21 is particularly adapted to being
displaced along its length in a manner similar to that used to
displace movable aperture plates 18 and 26.
Affixed to the upper portion of the movable aperture plate 21 is a
screw thread follower 86. The screw thread follower 86 engages the
threads of the lead screw 87. The lead screw 87 runs parallel to
the movable aperture plate 21 and into bearing assemblies in the
receiver collimator 71 near each end. An one end of the lead screw
87 there is affixed a sprocket 88 in engagement with a chain 89
which is in further engagement with a sprocket 91 fixedly attached
to the output shaft of motor 22. In the exemplary embodiment, the
motor 22 is preferably an electric gear motor.
At the opposite end of the lead screw 87 there is fixedly attached
a sprocket 93 in engagement with a chain 94 which further engages
sprocket 92 fixedly attached to the output shaft of a potentiometer
14. The potentiometer 14 is utilized to provide a shaft position
feedback to the control circuitry of the motor 22 as described in
more detail below. Attached to the lower portion of movable
aperture plate 21 are two linear bearng blocks 96 which ride on a
shaft 97 which is affixed to each side of the collimator housing
71.
There is thus provided a means for displacing the movable aperture
plate 21 along its length by electrically actuating the motor 22 to
cause the lead screw shaft 87 to rotate, which in turn causes the
movable aperture plate to be displaced via the screw thread
follower 86. The potentiometer provides an electrical feedback to
the control circuitry indicative of the position of the movable
aperture plate 21. When the operator elects to use either an 8
millimeter or 13 millimeter slice thickness, the select switch is
indexed to the appropriate setting. This action permits the output
of the potentiometer 14 to cause the motor 22 to start, rotate
clockwise or counterclockwise and stop at a predetermined position.
The motor 22 thus turns the lead screw thereby moving the screw
thread follower 86 and the attached movable aperture plate 21 to
either a masked or an unmasked position as more fully described
below. It should be noted that neither the fixed aperture plate 19
nor the movable aperture plate 21 affect in any way the width of
the X-ray beam.
Near the exit end of the collimator 17 are three aperture plates
designated 18, 23, and 26. The center aperture plate 23 is a fixed
aperture plate having a series of 24 rectangular openings therein
as more fully illustrated in FIG. 15. The openings in the fixed
aperture plate 23 are arranged in two rows of 12 openings similar
to the arrangement of fixed aperture plate 19. The openings in the
fixed aperture plate 23 are greater on all sides than the actual
beam size and function only to prevent X-rays from passing around
the two movable aperture plates 18 and 26 adjacent to it, which
plates are smaller than the opening in the collimator space in the
housing 71.
The movable aperture plates 18 and 26 have a means similar to that
of movable aperture plate 21 for causing the displacement thereof.
In particular, associated with movable aperture plate 18 is lead
screw 101, and screw thread follower 102. Along the lower portion
of the movable aperture plate 18 are linear bearing blocks 103 and
104 which ride on fixed shaft 105. The lead screw 101 rides in
bearings affixed in each side of the housing 71 and extends
therethrough. Attached to one end of the shaft is a gear 111 and a
chain coupling 112, which coupling mechanically couples the lead
screw 101 with the output shaft of gear motor 24. Similarly
attached to movable aperture 26 is a screw thread follower 115
which engages lead screw 116 which is similarly disposed in each
side of the housing. Affixed to one end of the lead screw 116 is a
gear 117 which is in engagement with gear 111. The bearing blocks
118 and 119 are affixed to the lower part of the movable aperture
plate 26 and move along shaft 121 which has its ends attached to
the housing 71. It should also be noted there is affixed to the
opposite end of the lead screw 101 a gear 125 which is in
engagement with gear 127 which is fixedly attached to the output
shaft of the potentiometer 13.
It may now be appreciated that when the motor 24 is electrically
actuated, the lead screw 101 is driven via the coupling 112 in one
direction while the lead screw 116 will be driven via gears 111 and
117 in the opposite direction. The effect of this motion will cause
the openings in the two movable apertures 18 and 26 to move either
into or out of alignment thus creating a larger or smaller aperture
opening for the X-ray beams to pass through to thereby change the
beam width. This action may be more fully appreciated by referring
to FIGS. 14 through 16 which illustrate the aperture plates 18 and
26. It may be noted that opposite relative displacement of the
movable aperture plates 18 and 26 along their length will cause the
edges of the movable aperture plates to be displaced to thereby
cause an increase or a decrease in the effective aperture opening
of the group of plates comprising variable aperture plate 18, and
movable aperture plate 26. It should be noted that the equal
relative displacement of plates 18 and 26 causes a variation in
beam width without changing the relative position of the center
line of each beam.
The potentiometer 13 provides an electrical feedback to the control
circuitry as described below of the position of the movable
aperture plates. The desired slice width is selected remotely by
means of a control switch, and as previously described for the
aperture plates 19 and 21, the movable aperture plates 18 and 26
are automatically positioned by control circuitry more fully
described below.
Referring now to FIG. 17 of the drawings, there is disclosed the
circuitry provided for controlling the movable aperture plates
contained within the source and receiver collimators 33 and 17,
respectively. The thickness select switch 111 is utilized by the
scanner operator to selectively cause the movable aperture plates
within the source and receiver collimators to be displaced so that
the thickness of the X-ray beams transmitted through the source and
receiver collimators may be varied.
In the exemplary embodiment, two beam thicknesses are optionally
provided. A first beam thickness is provided during a first state
when the movable aperture plate 38 is displaced such that it is not
disposed within the beam path and the movable aperture 21 has the
openings thereof in alignment with the large openings in the fixed
aperture plate 19 so that the shape of the X-ray beams transmitted
through the movable aperture plate 21 is substantially determined
by the aperture plate 19.
When it is desired to decrease the thickness of the X-ray beams
utilized by the scanner, the thickness select switch 111 is
actuated to cause the movable aperture plates 38 and 21 to be
displaced in a manner such that the movable aperture plates 38 and
21 shape the X-ray beam. Specifically, when the thickness select
switch 111 is switched from a first to a second state, a signal
corresponding to the second state is transmitted to the solenoid
drive circuit 112, which in turn energizes rotary solenoid 28 to
cause a displacement of the movable aperture plate 38 to a position
adjacent fixed aperture plate 29. This position is more fully
disclosed in FIGS. 2 and 3 of the drawings.
In addition, when the thickness select switch 111 is switched to
the second state, a signal corresponding thereto is also
transmitted to a thickness selector means 114. The thickness
selector means 114 may be comprised of an analog multiplexer that
selects one of two reference voltages from a voltage source 119 and
causes the selected reference voltage to be outputted to a position
comparator 115. The position comparator 115 has a second input
means for receiving an input from the potentiometer 14. The output
of the potentiometer 14 is a voltage level which varies in
accordance with the position of the movable aperture plate 21.
The position comparator 115 will compare the selected voltage from
the thickness selector means 114 to the voltage output provided by
the potentiometer 14 and will generate an error signal when a
difference exists between the potentiometer output voltage and the
selected reference voltage. A positive output error signal will be
provided by the position comparator 115 when the selected reference
voltage is smaller than the potentiometer output voltage, and a
negative error signal will be provided when the selected reference
voltage is larger than the potentiometer output voltage. In the
exemplary embodiment, the first reference voltage is +4.5 volts and
the second reference voltage is +3.0 volts. The first and second
reference voltages are associated with the first and second states,
respectively. Because the gain of the position comparator 115 is
typically a gain of 10, a large error voltage of plus or minus 12
volts will typically be provided at the output of the position
comparator 115 when a change in the switch setting has been
made.
The error signal goes from the position comparator 115 to
bi-directional motor drive circuit 121 via amplifier 125 and to
error comparator 116. Normally, an input to the motor drive 121
causes the motor 123 to rotate and thus causes the movable aperture
plate 21 to be displaced as disclosed above. However, a motor drive
clamp circuit 126 is provided which is used to selectively disable
the motor drive 121.
The motor drive clamp circuit 126 is controlled by the output from
error comparator 116. The error comparator 116 is responsive to
both positive and negative error signals. The output of the error
comparator 116 will toggle between plus and minus 12 volts
depending on the level of its input signals. When the thickness
select switch 11 is at a preset position, the output of the error
comparator 116 is minus 12 volts because the motor drive clamp
output feedback 129 maintains a minus 1 volt signal at the
non-inverting input of the error comparator 116, and the error
signal provided by position comparator 115 is near zero volts. The
error signal from comparator 115 must therefore exceed a magnitude
plus or minus 1 volt in order to toggle the error comparator 116.
This condition occurs only when a new reference voltage
corresponding to a new beam is selected by the thickness select
switch 111.
When a new beam thickness is selected, an error signal of
approximately 12 volts will be outputted by the position comparator
115 to cause the error comparator 116 output to switch to plus 12
volts which will drive the motor drive clamp 126 output to minus 12
volts. The minus 12 volt output of the motor drive clamp 126
produces two results. First, motor clamping is removed from the
motor drive; and second, the minus one volt from the motor drive
clamp 126 is changed to 0.1V to the error comparator 116 input.
This condition allows the movable aperture 21 to gradually approach
the desired position where motor clamping is re-applied.
With motor clamping removed, the motor drive circuitry is free to
respond to error signals generated by the position comparator 115.
The motor drive circuitry includes an error amplifier and a
bi-directional motor drive which applies positive or negative
voltages to the motor 22.
As the movable aperture 21 is driven into the proper position by
the motor 22, the error signal voltage from the position comparator
115 will gradually reduce. When the error signal voltage falls
below 800 millivolts, the motor drive circuitry will begin to
reduce motor speed to prevent overshooting the desired position.
The error signal will continue to decrease until about 100
millivolts. The error comparator 116 will then remove the plus 12
volts to the motor drive clamp 126 which will cause the output of
the motor drive clamp 126 to inhibit the motor drive 121 and also
change signal 129 from 0.1V back to 1.0V thus increasing the
magnitude of the error necessary to turn the circuit back on to
.+-.1.0V. With the movable aperture thus locked in position by the
motor clamp circuitry 126, a new image thickness selection is
required before the motor 22 will again move.
Similar control circuitry is provided for the drive motor 24 which
causes the relative displacement of movable aperture plates 18 and
26. As can be seen from FIG. 17, there is provided a width selector
switch 131 which provides an electrical signal to the width select
means 134 in accordance with the particular width selected. The
width select means 134 is, in the exemplary embodiment, an analog
multiplexer similar to that described above which selects one of
two reference voltages from a voltage supply and provides one of
the reference voltages to a position comparator 135 which also
receives an input from potentiometer 13 in a manner similar to the
control circuitry described above.
Reference voltage supply 152 consists of two potentiometers, one
for each position. Adjustment of these potentiometers provides
independent infinitely variable adjustment of the two reference
voltages. The availability of infinitely variable inputs to the
width select means 134 allows the movable aperture plates displaced
by the motor 24 to provide infinitely variable beam widths.
It can not be readily appreciated that the error comparator 135,
motor drive clamp 146, motor drive 141, and motor 24 function in a
manner analogous to the corresponding elements of the control
circuit described above to provide a means for preselecting one of
a plurality of beam width selections, which may be infinitely
variable, to automatically cause the movable aperture plates 18 and
26 to be displaced relative to each other in order to vary the
shape of the X-ray beam transmitted through aperture plates 18, 23
and 26 in the manner described above.
It should be appreciated that the exemplary embodiment described
herein utilizes a source collimator having movable aperture means
which provide only two beam shapes. It should be realized, however,
that the source collimator may have a plurality of movable
apertures to afford a greater degree to control of beams of the
X-ray transmitted through the source collimator. It should be
further appreciated that while the movable aperture plates of the
instant invention are intended to be displaced to a limited number
of set displacements, the modifications necessary to allow the
movable apertures to be displaced in an infinitely variable manner
would be obvious to one skilled in the art.
The following claims are intended to cover all modifications which
do not depart from the spirit and scope of the invention. The
invention is not to be necessarily limited to the specific
construction illustrated and described, since such construction is
intended to be illustrative of the principle of operation and the
means presently devised to carry out said principle. It is to be
considered that the invention comprehends any minor change in
construction that is permitted within the scope of disclosure.
* * * * *